Infectious illness causes an increase in the breakdown of body tissues. The extent of body protein loss depends on the severity and duration of illness, particularly the presence of fever, the degree of loss of appetite and resultant reduction in food intake, and the initial nutritonal status of the patient. Although this metabolic response to infection has been considered to be only detrimental to the host, a better understanding of this response has identified specific components, such as the increased synthesis of acute phase proteins and the development of humoral and cellular immunity which are beneficial, despite the net loss of body protein. By the use of newer techniques that enable the investigator to determine the dynamics of body protein metabolism using tracer studies with radioactive and non-radioactive substances, advances into the understanding and therapeutic manipulation of the metabolic respone to infection can be expected. Our studies are of the hormonal, metabolic and dynamic protein response to mild infection (yellow fever immunization) and in spontaneously acquired infections in hospitalized patients receiving conventional diets that contain different amounts of protein and calories. Our aim is to determine the optimal diet composition that will maximize the beneficial effects while minimizing the adverse effects of infection on protein metabolism.